Corona shielding



T. F. PETERSON CORONA SHIELDING April 16, 1957 2 Sheets-Sheet 1 Filed May 29, 1952 FIG.

FIGJZ ATTORNEY pri 1957 T. F.- PETERSON CORONA SHIELDING 2 Sheets-Sheet 2 Filed May 29, 1952 FIG. 6

FIG. 7

FIG; 9-

INVENTOR YTHOM-AS F. PETERSON,

I BY 2 ATTORNEY United States Patent CORONA SHIELDING Thomas F. Peterson, Shaker Heights, Ohio Application May 29, 1952, Serial No. 290,639

6 Claims. (Cl. 174-73) This invention relates to the shielding of suspended highvoltage electric cable structures from corona and its effects. Corona glow discharges take place at the interface between the structure or electrode and the surrounding air or gas when the electric field strength in the gas reaches a value at which ionization becomes copious, and a glow appearing as bluish tufts or streamers develops at irregularities, such as points, sharp corners, etc.; as the voltage increases, the volume of gas in corona increases, and ultimately spark-discharge and arc-over may result. The voltage drop through the gas in corona is less than that before corona; but, since both charging and in phase currents pass through this region, a wattage power loss develops as a result. During discharge in air, ozone and nitrous acid are usually formed with their corrosive effects; and, moreover, the discharges produce high frequency components in the voltage wave to interfere with radio transmission.

The customary approach to the suppression of corona and its effects is that of reducing surface stress at irregularities, where concentrations of charges occur, by means of clamped on metallic discs, tubing, spheres, etc., that increase the effective conductor diameter and thereby reduce the electrical stress, i. e., potential gradient, at the gas and metal interface in inverse ratio to the increase of radial dimension. But questions of cost, weathering conditions, vibration fatigue, etc., enter into the solution of the problem; the designs, accordingly, involve compromises suited only for average conditions and not for exceptional conditions, such as heavy rain, low barometric pressure, high altitudes, etc.

The present invention provides a different approach to the elimination of corona and its many disturbing effects. Its departure from the prior art depends essentially upon a shielding having incorporated therein a non-metallic, i. e., organic, semi-conducting material in series electrically between the cable structure or high potential metallic electrode and the contacting gas, and having a smooth, continuous surfacing of rounded canvexity and enlarged diameter relative to the electrode at the interface between the electrode and the gas in the zone of high stress.

Such material of semi-conducting nature is conveniently prepared from natural rubber or synthetic highpolymer plastics (see Patents 2,322,702 and 2,446,387) by having incorporated therein conducting carbon particles (acetylene black). The plastics in themselves are high in insulating value; but, when modified with conducting carbon black, their electrical characteristics can be markedly altered, and resistivities ranging from about ohms to thousands of megohms per cm. cubed obtained by varying the proportion of carbon black. The preferred proportions of carbon black are from about 35 to 50 percent of the total composition; within this range resistivities of hundreds of thousands down to about one hundred ohms are possible.

The semi-conducting compositions serve surprisingly Well as distributed resistors in series between the elecr 2,789,154 Ice Patented Ap 16, 1957 trade and the surrounding gas to dampen or reduce corona potency and also to eliminate harmonic high frequency waves that interfere with radio reception. The compositions are, moreover, characterized by a low work function (in contrast to a metal which has a high work function) so that for a given potential gradient in the gas the discharges are relatively much less severe; and the compositions show a remarkable stability over long periods of time under severe test conditions. A further advantage is that, in the event any power arc-over should take place to a shield, the material being organic is burned or destroyed to not only increase the length of the arc path but provide as well visible evidence of the damage for repair or replacement. An outstanding ad vantage, also, is that normally the units can be preformed to shape before application, for the elastic nature of the material permits a unit to be sprung into place; this allows for assembly on an existing cable structure without introducing new sources of corona, such as bolts, nuts, etc.

The invention is capable of numerous embodiments, of which some are illustrated in the accompanying drawing, in which drawing Fig. 1 shows a shielding applied on location to the terminal of a preformed armor on a cable;

Fig. 2 shows in radial cross-section a pre-molded toroid or split doughnut in place on a cable at the end of a preformed armor;

Fig. 3 is an end view of Fig. 2;

Fig. 4 shows a disc shielding with a spherical metal surfacing;

Fig. 5 is a cross-section on line 5-5 of Fig. 4;

Fig. 6 shows in axial cross-section a split spherical or rounded shielding cemented in place;

Fig. 7 shows in axial cross-section a split sphere held in location by a retainer spring;

Fig. 8 shows a helically-molded shielding that can be turned or screwed over a projection;

Fig. 9 illustrates a form of shielding spanning a suspension clamp; and

Fig. 10 is a cross-section on line 10-10 of Fig. 9.

In the Figures 1 to 8 the shielding is illustrated in conjunction with armon'ng on a cable, which armoring is generally made of preformed wires or rods that are twisted into place on a cable. It is, however, applicable wherever projections exist on a cable structure of a kind that cause stress concentrations.

Referring to Fig. l, the terminal protection takes the form of a molded rounded mass 11 of rubber, neoprene, or other organic insulating media made semi-conducting throughout the mass by means of conducting carbon particles; instead of the entire mass of insulation being made semi-conducting, a surfacing layer of sprayed latex or the like incorporating conducting carbon particles can be applied; or the rounded mass can be built up in whole or in part of semi-conducting tape wound about the armoring 12 and the conductor or cable 13. The resiliency of the material and its mounting, and its relatively light weight in comparison with metal, gives rise to excellent vibration characteristics; mechanical stress .at the point of attachment is negligible, and failure of the main structure due to vibration reflections is prevented.

Fig. 2 and Fig. 3 illustrate a premolded or preformed shielding of toroid or doughnut shape made of flexible material as rubber and the like, which can be snapped into position. For this purpose the structure 21 is provided with a slit 22 so that the toroid can be opened and pushed over the cable or conductor 23. By press ing the toroid against the armor terminal 24, the accumulation of stress at the projection formed by the terminal is prevented.

In Figs. 4 and 5 the shielding takes the form of a disc spacer 31 made of semi-conducting material. The spacer is split as at 32, and the central opening 33 is of a size to grip the conductor; with this construction, the spacer can be slipped radially over the conductor to grip the latter. A continuous rounded outer surfacing is supplied by a hollow spherical member subtending the armor 34 and the conductor 35 and engaging the disc 31; the sphere can be of conducting sheet metal and can consist, for example, of two halves that are crimp-locked together as at 36.

In Fig. 6 is shown a split spherical member 41 of sponge rubber, for example, that can be semi-conducting throughout or insulating and given a semi-conducting coating of latex, for example, that contains "conducting carbon particles; or. the gas-interface surfacing can be one of metal or a non-metallic weather-resistant layer so long as there is included in series between the cable conductor and gas a distributing resistor of semi-conducting character. The spherical sections can be cemented together and to the conductor to retain the sphere in position. The member, however, need not be a true sphere but can be any solid of revolution generated by a suitable curve, such as an ellipse, a sine curve, a cycloid, or approximations of such curves; the term spherical-like is intended to include these various forms.

A modification of Fig. 6 is shown in Pig. 7, wherein a groove or indentation 51 is formed in the outer surface of the structure for a spring retainer 52, e. g., formed of stainless steel, to take the place of or in addition to cementing the shielding sections.

In Fig. 8 the embodiment takes the form of a helically coiled member 61, preferably of increasing diameter from the ends 62 toward the middle 63, as shown, and molded of semi-conducting rubber, neoprene or the like; the convolutions are closely contacting so as to present an outer enveloping surface of rounded character. By slipping one end 62 of the element 61 over the cable structure, the Whole can then be screwed on the structure 65 and over the projection 66 to form a rounded shielding over the projection.

Figs. 9 and 10 illustrate an embodiment for applic tion particularly at a suspension clamp. A shielding element 71 is shown, consisting of the inverted U-shaped of hook sections 72, 73 connected by a longitudinally extending section 74; the sections are proportioned to bring the hook ends 72, 73 at the junctions 75 of the cable 76 and the clamp 77. The shielding can be of semi-conducting rubber or equivalent, and it can be coated with metal paint, foil or the like. Any arc-over 4 that may occur may destroy the shield; but, as before indicated, this presents advantages, such as increasing arc length, making any damage visible, etc. In this form, the shielding does not cii'cum-ferentially encircle the cable structure but xtends only in the locations of stress concentrations.

What is claimed is:

i. The combination with a high tension electrical con- T ductor, suspended in a surrounding gas and having at a point intermediate its extremities a projection causing a discontinuity in the conductor diameter, of a corona shielding element mounted on the conductor and about the projection in the region of discontinuity to enlarge the effective surface area of the projection for minimizing any corona discharge, said element having a smooth and rounded surface and comprising essentially a molded non-metallic and resilient plastic mass of insulating character having incorporated therein conductive particles in amount to provide an electrical resistivity within the range of several ohms to thousands of megolnns per centimeter cubed.

2. Corona-shielding element accordin to claim 1 with a resistivity range of from about a hundred to hundreds of thousands of ohms per cm. cubed.

3. Corona-shielding element according to claim 1 in which the element is a preformed toroid for encircling the cable structure.

4. Corona-shielding element according to claim 1 in which the element comprises a disc-shaped spacer of the non-metallic plastic mass and a hollow spherical metallic member about the spacer and the discontinuity.

5. Corona-shielding element according to claim 1 in which the element comprises a spherical-like member split into sections for application, and means for securing said sections together.

6. Corona-shielding element according to claim 1 in which the element comprises a spherical-like member split into sections, and a retaining ring for securing said sections together.

References Cited in the file of this patent UNITED STATES iA'i'ENTS 1,157,344 Thomson Oct. 19, 1915 2,142,625 Zoethut ian. 3. 1939 2,379,942 Webber July 10, 1945 2,436,306 Johnson Feb. 17, 1948 2,440,828 Nichols May 4, 1948 onah-w 1-. 

